1. Field of the Invention
The present invention relates to a superconducting filter for radio-frequency signals.
2. Description of the Related Art
Various radio-frequency filters are used at mobile communication stations, etc. which treat signals of a some GHz frequency region. As the reception filter of the radio-frequency filters used in the mobile communication stations, etc., coaxial resonator-type, dielectric resonator-type, superconducting resonator-type, etc. are known. The reception filters of these types are required to realize downsizing and higher frequency selectivity.
The superconducting-type reception filter including as the circuit conductor a superconductor of an oxide high temperature superconductor or others can provide high no-load Q, which is advantageous in high frequency selectivity. On the other hand, as for the transmission filter, which treats large electric power, the superconducting-type cannot easily make downsizing and good electric power characteristics, etc., such as power resistance, etc. compatible with each other. The compatibility between both is a large problem.
In the downsizing, the filter of planar circuit-type is superior to the dielectric resonator-type, the coaxial resonator-type, etc. Furthermore, in the frequency region of below some GHz, where the mobile communication is relatively advantageous, the planar circuit-type filter using superconductor film of good YBCO, etc. can provide high no-load Q which is higher by places than the ordinary resonators using normal conductor film of, gold, silver, copper, etc., and can ensure high frequency selectivity.
In trying to downsize the planar circuit-type superconducting filter, the following methods have been so far studied. For example, the method of bending and deforming superconductor film line patterns to thereby decrease the area of a region where the resonator pattern is to be formed. The method of using a substrate of high dielectric constant as a substrate for resonator pattern conductors to be arranged to thereby increase the effective dielectric constant has been studied.
For the planar circuit-type superconducting filter, in trying to downsize the filter and improve the power characteristics as a power application, the following method has been studied. For example, the superconductor pattern of the resonance circuit is in circular, polygonal or other patches to thereby mitigate the current density concentration by TM mode or others has been studied. The method of controlling the grain boundary, the impurity or others of oxide high temperature superconductor film to thereby develop better oxide high temperature superconductor film to be used as the circuit conductors has been studied.
Furthermore, the method of using a hybrid structure of the planar circuit type and dielectric substances except the dielectric substances of the substrate to thereby mitigate the concentration of current density on the superconductor has been studied.
Non-Patent References 1 to 3 listed below disclose the techniques of forming planar circuits, such as coplanar circuits, microstrip line circuits, etc., using oxide high temperature superconductor films such as copper oxide high temperature superconductor films to thereby form passive circuits, such as radio-frequency filters, etc.
For the reception radio-frequency filters of the superconducting filters including oxide superconductors, it is an important problem to be downsized as much as possible. For the transmission radio-frequency filters treating high power, it is an important problem, in addition to downsizing, to improve the power characteristics as much as possible.
Following references disclose the background art of the present invention.
[Patent Reference 1]
Japanese published unexamined patent application No. 2002-57506
[Patent Reference 2]
Japanese published unexamined patent application No. 2003-332812
[Patent Reference 3]
Japanese published unexamined patent application No. 2000-269704
[Patent Reference 4]
Japanese published unexamined patent application No. Hei 11-261307 (1999)
[Patent Reference 5]
Japanese published unexamined patent application No. 2002-141706
[Patent Reference 6]
Japanese published unexamined patent application No. 2001-267806
[Patent Reference 7]
Japanese published unexamined patent application No. 2000-212000
[Patent Reference 8]
Japanese published unexamined patent application No. Hei 10-224110 (1998)
[Non-Patent Reference 1]
M. Hein, High-Temperature-superconductor Thin Films at Microwave Frequencies, Springer, 1999
[Non-Patent Reference 2]
Alan M Portis, Electrodynamics of High-Temperature Superconductors, World Scientific, 1992
[Non-Patent Reference 3]
Zhi-Yuan She, High-Temperature Superconducting Microwave Circuits, Artech House, 1994